Two switch high voltage modulator for plasma-based ion implantation

A new pulse modulator for plasma-based ion implantation (PBII), which is adaptable for a wide range of the load impedance has been developed, using a newly-set closing switch, which is set in parallel with the target and grounded chamber. In this article, time evolutions of the voltage applied to a target and current through the target for a conventional modulator with only a main switch, are presented, where the diameter of a disk target and the target position from the plasma source are varied. The output impedance of the modulator changes the rise time of the applied voltage due to the charging of the stray capacitor in the circuit. At the end of the target voltage, the stray capacitor begins to discharge to the equivalent resistance of the ion sheath. This brings a decay of the target voltage with a time constant consisting of the total capacitance and the sheath resistance. For the newly-developed modulator, the output impedance of the modulator was low enough to decrease the rise time of the target voltage. The sheath-capacitance was shortened by a new closing switch to promote the discharge. Due to both the low output impedance of the modulator and a newly-set closing switch, the modulator output voltage can be transferred to the target as a load without change of the waveform.